Natural gas and crude oils produced in certain areas of the world contain mercury in quantities sufficient to render their processing problematical. For example, hydrocarbon condensates derived from natural gas produced in certain regions of southeast Asia may contain over 1000 parts per billion by weight (ppbw) of mercury, while crude oils from certain regions of Argentina frequently contain well over 2000 ppbw of mercury. The produced waters from gas and oil wells with elevated levels of mercury may also contain high levels of mercury precluding their discharge to the environment as the direct result of contact between the water and the oil or gas in the subterranean production interval. Wastewater streams associated with processing the gas and oil may also contain mercury arising from contact between process water streams and hydrocarbon streams. The contact may take place, for example, by the use of water or aqueous treatment streams to remove other contaminants such as nitrogenous compounds.
The mercury may be present in several forms including ionic, elemental, particulate and organic. Crude oils, for example, may contain elemental mercury but this may be oxidized in various process units to produce water-soluble salts (Hg+, Hg2+) and complexes. Additionally, anaerobic bacteria can convert suspended mercury in particulate forms to water-soluble organic forms so that transfer between the two solvent species occurs readily.
The presence of the mercury raises problems of two kinds. First, the mercury may attack the metals in processing equipment by the formation of amalgams; this is a problem which is especially notable with items made of aluminum and aluminum alloys, such as the cold boxes in cryogenic plants such as the ethylene separators found in petrochemical units and in natural gas treatment installations. The presence of mercury on the equipment may also dictate its treatment as hazardous waste when removed from service. Mercury poisoning may also reduce the life of processing catalysts.
Second, mercury, as an elemental impurity that cannot be destroyed but only moved from one stream to another, will often enter process water streams. This may occur by direct contact with the stream, for example, during washing or from the use of process steam. Recent studies have shown that as much as 20% of the mercury in the crude can enter a refinery wastewater stream. In any event, increasingly stringent environmental regulations make it necessary to remove the mercury from the water before it can be discharged to the environment. Recent environmental limits may place the discharge target as low as 0.1 ppbw. The difficulty in dealing with mercury-containing crudes and other petroleum streams is exacerbated by the fact that increasingly, shortages of high quality crudes have led to the use of supplies of crude containing high levels of mercury.
Currently, few technologies are available for removing mercury from streams of wastewater and produced water. The main commercial technology available for treating mercury in water consists of adding one of several commercially-available precipitants, usually sulfided polymers, to precipitate dissolved ionic mercury and remove it by means of gas or air flotation. A technique of this kind is described in U.S. Pat. No. 6,635,182 (Coleman). Although this method is effective at removing the bulk of mercury found in wastewater (ionic species as Hg2+), it cannot remove all mercury species which may be present, including insoluble particulate mercury compounds, elemental mercury (Hg(0)), present either as such or dissolved in minor amounts in the water, and organic mercury, principally monomethyl and dimethyl mercury. Where significant amounts of mercury or numerous different species are present and effluent limits are low, existing technologies are not likely to remove the amounts of mercury necessary to achieve environmental compliance.
Other proposals for treating aqueous streams to remove mercury and other heavy metals are found in U.S. Pat. No. 4,814,091 (Napier), U.S. Pat. No. 5,667,694 (Cody), U.S. Pat. No. 6,165,366 (Sarangapani) and U.S. Pat. No. 7,092,202 (Zhuang). Prefiltration followed by pH adjustment and sulfide precipitation followed by flocculation and posifiltration is used in the method of U.S. Pat. No. 4,814,091. The method described in U.S. Pat. No. 5,667,694 uses an organoclay sorbent which can then be separated from the water, containing the removed metal. A treatment better adapted to continuous use is described in U.S. Pat. No. 6,165,366, which uses sequential hypochlorite oxidation, filtration and removal of organics using activated carbon. In the method described in U.S. Pat. No. 7,029,202, a lignin derivative is used initially to form a complex compound with the mercury or other metal after which a coagulant is used to form a floc which is then separated as a sludge. These methods have, however, not shown themselves to be sufficient to remove mercury in many wastewater stream to the levels needed for regulatory compliance.